Environmental Engineering Reference
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Fig. 3 Crystal structures of
various Na
x
MO
y
: a P2-
Na
x
CoO
2
b O3-Na
x
CoO
2
c
P3-Na
x
CoO
2
, and d
Na
0.44
MnO
2
(Na: yellow, Co/
Mn/V: blue,O:red)[
2
]
Though LiCoO
2
is the hottest commercial cathode material for Li-ion battery,
Na
x
CoO
2
unfortunately cannot match a good cathode either in energy density or in
cyclability for Na ion batteries. Besides, Co as a precious metal does not meet the
low-cost demand of Na-ion battery.
3.1.2 Na
x
MnO
2
In the early 1970s, Mn-O-Na ternary system has been widely investigated and a
wide structural Na
x
MnO
2
(depending strongly on the concentration of Na and the
preparation condition) has been reported [
11
]. The most typical structures are a-
NaMnO
2
with lamellar structure of O
0
3 type (Fig.
3
a), Na
0.7
MnO
2
with lamellar
structure of P2 type (Fig.
3
b), and Na
0.44
MnO
2
with three-dimensional channel
structure (Fig.
3
d). Delmas et al. studied the Na intercalation behavior in the above
three types of Na
x
MnO
2
. They found that within a certain range of x
(0.45 B x B 0.85 for a-NaMnO
2
and Na
0.70
MnO
2
, 0.30 B x B 0.58 for
Na
0.44
MnO
2
), these materials can maintain very well their pristine structures,
showing potential to be used as cathode materials for Na-ion batteries. Caballero
et al. [
12
] prepared P2-Na
0.6
MnO
2
by using sol-gel method, which showed high
purity and a well-defined layered structure (Fig.
3
b). The material delivered a high
specific capacity of ca. 140 mAh g
-1
(corresponding to an intercalation amount of
~0.52)
during
the
initial
several
cycles,
and
then
the
capacity
declined
on
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